Carrier Density Induced Metal-Insulator Transition in Bi2O2Se Nanosheets

Abstract

The metal-insulator transition (MIT) in two-dimensional (2D) systems fascinates researchers by challenging existing theories regarding its microscopic origins. In 2D materials, MIT emerges from the interplay of disorder and carrier interactions, modulating the delicate balance between localization and delocalization of charge carriers. Here, we report MIT in few-nanometer-thick Bi2O2Se nanosheets by regulating electron density via back-gate voltage. At high carrier density, conductivity exhibits a near-linear scaling with density (σ ∝ n2Dα with α ≈ 1), reflecting transport dominated by screened Coulomb impurity scattering. As the electron density decreases below a critical threshold (n2D < nth), charge homogeneity collapses, leading to strong spatial inhomogeneities that trigger a percolation-driven MIT. In this insulating regime, isolated conducting puddles fail to connect, and charge transport follows thermal activation of carriers trapped in localized states, highlighting the significant role of disorder in this material. We found the average value of percolation exponent δ = 1.28, which matches well with the theoretically predicted value of 1.33 for 2D continuum percolation. Unlike Anderson localization, which predicts universal localization in 2D with any disorder and requires significant scattering, percolation in high-mobility Bi2O2Se is driven by tunable carrier density and impurity scattering. These findings show that the gate-tunable MIT in Bi2O2Se nanosheets is largely controlled by percolation, offering a simpler way to understand conduction in 2D materials where disorder is moderate, and carrier density plays a key role.